Typical procedures for analyzing biological materials, such as nucleic acid, protein, lipid, carbohydrate, and other biological molecules, involve a variety of operations starting from raw material. These operations may including various degrees of cell separation or purification, cell lysis, amplification or purification, and analysis of the resulting amplification or purification product.
As an example, in DNA-based blood analyses samples are often purified by filtration, centrifugation or by electrophoresis so as to eliminate all the non-nucleated cells, which are generally not useful for DNA analysis. Then, the remaining white blood cells are broken up or lysed using chemical, thermal or biochemical means in order to liberate the DNA to be analyzed. Next, the DNA is denatured by thermal, biochemical or chemical processes and amplified by an amplification reaction, such as PCR (polymerase chain reaction), LCR (ligase chain reaction), SDA (strand displacement amplification), TMA (transcription-mediated amplification), RCA (rolling circle amplification), and the like. The amplification step allows the operator to avoid purification of the DNA being studied because the amplified product greatly exceeds the starting DNA in the sample.
If RNA is to be analyzed the procedures are similar, but more emphasis is placed on purification or other means to protect the labile RNA molecule. RNA is usually copied into DNA (cDNA) and then the analysis proceeds as described for DNA.
Finally, the amplification product undergoes some type of analysis, usually based on sequence or size or some combination thereof. In an analysis by hybridization, for example, the amplified DNA is passed over a plurality of detectors made up of individual oligonucleotide detector fragments that are anchored, for example, on electrodes. If the amplified DNA strands are complementary to the oligonucleotide detectors or probes, stable bonds will be formed between them (hybridization). The hybridized detectors can be read by observation using a wide variety of means, including optical, electromagnetic, electromechanical or thermal means.
Other biological molecules are analyzed in a similar way, but typically molecule purification is substituted for amplification, and detection methods vary according to the molecule being detected. For example, a common diagnostic involves the detection of a specific protein by binding to its antibody. Such analysis requires various degrees of cell separation, lysis, purification and product analysis by antibody binding, which itself can be detected in a number of ways. Lipids, carbohydrates, drugs and small molecules from biological fluids are processed in similar ways. However, we have simplified the discussion herein by focusing on nucleic acid analysis, in particular DNA analysis, as an example of a biological molecule that can be analyzed using the devices of the invention.
The steps of nucleic acid analysis described above are currently performed using different devices, each of which presides over one part of the process. In other words, known equipment for nucleic acid analysis comprises a number of devices that are separate from one another so that the specimen must be transferred from one device to another once a given process step is concluded.
To avoid the use of separate devices, an integrated device must be used, but even in an integrated device the biological material specimen must be transferred between various treatment stations, each of which carries out a specific step of the process described above. In particular, once a fluid connection has been provided, preset volumes of the specimen and/or reagent species have to be advanced from one treatment station to the next.
To this aim, various types of micropumps are used. However, existing micropumps present a number of drawbacks. For example, in the most commonly used micropumps a membrane is electrically driven so as to suction a liquid in a chamber and then expel it. Inlet and outlet valves ensure a one-way flow. Membrane micropumps suffer, however, from the fact that they present poor tightness and allow leakage. In addition, the microfluidic valves also leak and are easily obstructed. Consequently, it is necessary to process a conspicuous amount of specimen fluid because a non-negligible part thereof is lost to leakage. In practice, it is necessary to have available several milliliters of specimen fluid in order to obtain sufficient material for analysis. The use of large amounts of specimen fluid is disadvantageous both on account of the cost and because the processing times, in particular the duration of the thermal cycles, are much longer. In any case, imperfect tightness is clearly disadvantageous in the majority of applications and not only in DNA analysis equipment.
Other types of pumps, such as servo-assisted piston pumps or manually operated pumps, present better qualities of tightness, but currently are not integratable on a micrometric scale. Further common defects in known micropumps are caused by direct contact with the specimen undergoing analysis, which may give rise to unforeseeable chemical reactions, and high energy consumption.
EP-A-1 403 383 discloses a micropump formed in a first body of semiconductor material and comprising a plurality of fluid-tight chambers. The chambers have been sealed under predetermined low pressure or vacuum conditions and may be opened by electronically breaking the seal. The micropump is bonded on a second body, which accommodates an integrated biochemical microreactor and includes a microfluidic circuit filled with a biological sample. The micropump is arranged so that the chambers are fluidly coupled with the microfluidic circuit, once the seals have been removed. Since the pressure inside the chambers is lower than the external pressure, the biological sample is sucked toward the micropump. Sequentially opening the chambers thus results in controlled movement of the biological sample step by step along the microfluidic circuit. The volume of each chamber, the pressure level therein and the timing of the opening determine the flow of the biological sample.
The micropump of EP-A-1 403 383 may be bonded to microfluidic devices and overcomes any leakage problems. However, a separate semiconductor body and a separate manufacturing process are required, so that the micropump is still expensive and rather bulky. Moreover, bonding a finished micropump to a separate body incorporating a finished microfluidic circuit involves some critical matters, such as exact alignment of inlets of the vacuum chambers with ports of the microfluidic circuit. Misalignments may prevent fluidic connection between the (opened) vacuum chambers and the microfluidic circuit, thereby causing a failure of the microfluidic device.